Endocrinology of physiological and progestin-induced canine anoestrus

Abstract

The oestrous cycle of the domestic bitch is unique with regard to its considerable length compared with that of most other domestic animals. A non-seasonal anoestrus, with a duration that may last from 2 to 10 months, follows each oestrous cycle. The anoestrus can be prolonged by oestrus-preventing drugs such as progestins. The general aim of the studies in this thesis was to get further insight into the endocrinology of the physiological and the progestin-induced canine anoestrus. Studies performed throughout recent years have made clear that during the course of canine anoestrus many endocrinological changes take place at the hypothalamic, pituitary, and ovarian level. Apart from these changes, there are indications of involvement of dopaminergic influences. The results of the study described in Chapter 3 provide forceful evidence that the bromocriptine-induced shortening of the interoestrous interval is not triggered by a decline in plasma prolactin concentration. The results of the study described in Chapter 4 provides additional evidence that the premature onset of oestrus brought about by a dopamine agonist must be due to some other dopamine-agonistic effect, probably increased FSH secretion. The study in Chapter 5 is a report on the effects of GnRH administration on the plasma concentrations of reproductive hormones in intact and OVX bitches. The data presented in this study demonstrate that provocative testing of the pituitary-ovarian axis using GnRH may be helpful in differentiating between bitches in anoestrus and neutered bitches. The study reported in Chapter 6 describes the basal and GnRH-induced plasma concentrations of FSH and LH in four anoestrous and four OVX bitches. In this report indications were found that measurement of the plasma FSH concentration in a single plasma sample may prove reliable for determination of neuter status. Taken together, the results of the studies described in Chapters 5 and 6 provide a basis for the diagnosis of remnant ovarian tissue and verification of neuter status in the bitch. Chapters 7 and 8 are reports on the effect of MPA on canine adenohypophyseal function. The results of both studies demonstrate that treatment with MPA affects the hypothalamic-pituitary-ovarian axis. Oestrus, ovulation, and a subsequent luteal phase did not occur in any of the bitches during treatment with MPA. The prevention of oestrus by MPA cannot be ascribed to a significant reduction in circulating levels of either FSH or LH. On the contrary, during the first months of MPA treatment there was an increase in basal plasma FSH and LH, which may be due to a direct oestrus-preventing effect of MPA at the ovarian level. With continuing MPA treatment, basal plasma gonadotrophin concentrations returned to pretreatment levels and the pituitary FSH response to GnRH stimulation decreased, while several LH pulses were not accompanied by a significant FSH pulse, suggesting that MPA treatment attenuated pituitary FSH sensitivity to endogenous GnRH. The study in Chapter 7 also provides an integral picture of the effects of MPA treatment on adenohypophyseal function.